Gene flow is the transfer of genes from one population to another population, usually by migration of one population into the other population’s territory. Descriptively, it is also referred to as gene migration, gene admixture, interbreeding, or gene exchange between populations.
Almost always, gene flow refers to migration taking place primarily in one direction. It is necessary that one group not only move into another group’s area, but that the new group also reproduce with the group into which it migrates. Dispersal of one population into another population does not lead to gene flow if there is not reproduction between the two groups. In effect, new genes are introduced into the population into which the new group moves, and the two groups hence become more similar genetically over time.
Genetic drift (random changes in gene frequency within a small population, resulting in mutations) and sometimes natural selection tend to introduce more diversity between groups, but gene flow tends to decrease genetic differences between two groups. If gene flow is not opposed, over time, the two involved groups can develop a new genetic group, which can be unique, genetically located between the previous genetic composition of the two involved groups.
Gene flow can occur rapidly and can involve a large movement of one population into another population distantly located, sometimes with force involved, but history suggests that gene flow more likely has occurred with continuous slow movement of individuals of one population into another neighboring population. Contemporary human populations are relatively homogeneous genetically, suggesting that gene flow has been a major factor. Luigi Luca Cavalli-Sforza and his colleagues have shown that there is a strong relationship between geographic and genetic similarity (or, conversely, dissimilarity), suggesting that people frequently are genetically close to their neighbors. Gene flow is not the only reason for this, but frequently is a major reason.
Lack of gene flow also can characterize groups that have a small amount of genetic interchange with their neighbors, sometimes because one or more groups in an area want to remain separate identities. Eastern European Jewish males, for example, with an emphasis on maintaining their religion, averaged only about 1/2 of 1% of gene flow into their community per generation for about 50 generations.
Social relations between two groups can affect the speed and extent of gene flow. In the United States, for example, about 30% of people who identify as African Americans have White European male genetic backgrounds. This significant input of White European genes into the African American population is due largely to oppression of African Americans by European Americans, mostly of British background, under slavery. Native Americans also experienced significant gene flow, frequently following violent conquest, into their group by European Americans, mostly of Spanish background. Racial, ethnic, religious, and social class differences also can impede gene flow by discouraging reproduction between the groups.
References:
- Cavalli-Sforza, L. L., Menozzi, P., & Piazza, A. (1994). The history and geography of human genes. Princeton, NJ: Princeton University Press.
- Olson, S. (2002). Mapping human history: Discovering the past through our genes. Boston: Houghton Mifflin.
- Relethford, J. H. (2001). Genetics and the search for modern human origins. New York: Wiley-Liss.
